This invention relates to apparatus for sensing the position of a sheet along a feed path and, especially, to apparatus for sensing the movement of a sheet edge past a pair of feed rollers such as feed rollers of an electrophotographic copier.
In sheet-feeding apparatus it is often desirable to sense when the leading or trailing edge of a sheet has passed a given point. In particular, it is often necessary to sense either that the leading edge of a sheet has entered a pair of opposing feed rollers or that the trailing edge has emerged from a pair of feed rollers. It is common in the art to sense the presence of a sheet either by mechanical feelers which extend into the feed path and are pushed out of the way by a sheet or by photoelectric devices which sense the interruption of a radiant beam such as a light beam directed across the sheet path. All of these expedients suffer from one or more deficiencies which adversely affect reliability of operation.
Mechanical devices such as feelers that are delicate enough not to affect adversely the progress of the sheet being fed may not be as positive in opening or closing an electric circuit as desired. Photoelectric devices on the other hand, while not presenting an obstacle in the paper path, may in time operate unreliably either because of electrical component failure or because of buildup of dirt or grease which obstructs the light path.
Besides the problems of reliability, sensors of the type described above are not readily adaptable precisely to detect the movement of a sheet edge past the nip between a pair of feed rollers.